Inorganic glass has high transparency, heat resistance and dimensional stability and therefore has been used in a wide variety of industrial fields. Although inorganic glass has such excellent features, it has defects that it has high specific gravity and is easily broken as it has low impact resistance. Therefore, transparent plastics have been attracting attention in all the industrial fields from the viewpoints of lightweight and safety.
One of the application fields of the above transparent plastics is the field of spectacle lenses, and there are known spectacle lenses made of (meth)acrylate resin and polycarbonate resin.
One type of the above spectacle lenses is photochromic spectacle lenses. The photochromic spectacle lenses serve as sunglasses whose lenses are quickly colored outdoors where they are irradiated with light including ultraviolet radiation like sunlight and as ordinary transparent spectacles whose lenses are faded indoors where there is no irradiation, and demand for the photochromic spectacle lenses is growing nowadays.
As the method of manufacturing the above photochromic spectacle lenses, there are one in which a photochromic compound such as a chromene compound is dissolved in a monomer and the monomer is polymerized to obtain a photochromic lens directly (to be referred to as “kneading method” hereinafter), one in which a coating layer having photochromic properties (to be also referred to as “photochromic coating layer” hereinafter) is formed on the surface of a plastic having no photochromic properties (to be referred to as “coating method” hereinafter), and one in which a monomer having photochromic properties is poured into a space between a plastic lens and a glass mold to be polymerized and cured (to be referred to as “lamination method” hereinafter). The technologies of the above kneading method (WO01/005854 and WO09/075388), the coating method (WO11/125956 and WO03/011967) and the lamination method (WO01/005854) are proposed in these patent documents.
For these photochromic compounds and plastic optical articles comprising these compounds and having photochromic properties, the following properties are required from the viewpoint of a photochromic function: (I) the degree of coloration at a visible light range before ultraviolet light is applied (to be referred to as “initial coloration” hereinafter) should be low, (II) the degree of coloration upon exposure to ultraviolet light (to be referred to as “color optical density” hereinafter) should be high, (III) the speed from the stoppage of the application of ultraviolet light to the time when the compound returns to its original state (to be referred to as “fading speed” hereinafter) should be high, (IV) the repeat durability of the reversible functions of (II) and (III) should be high, (V) storage stability should be high, (VI) the compound should be easily molded into an optical article, and (VII) the mechanical strength of an optical article should be high.
With these technologies as background, there are proposed photochromic plastic lenses (optical materials) comprising a chromene compound which is hardly decomposed by light and rarely deteriorates in color development performance even when sunlight or light similar to sunlight is applied thereto continuously. The development of photochromic curable compositions comprising various polymerizable monomers and a photochromic compound (especially a chromene compound) in the kneading method and the coating method is now under way.
WO01/005854 and WO09/075388 disclose photochromic curable compositions comprising a specific (meth)acrylic polymerizable monomer having a (meth)acrylic group (general name for methacrylic group and acrylic group) and a chromene compound. Since the kneading method has a feature that photochromic plastic lenses can be mass-produced at a low cost by using a glass mold, it is now the major production method of photochromic plastic lenses.
However, the kneading method requires a matrix having excellent mechanical strength in order to obtain the strength of a lens substrate and has a problem that photochromic properties must be sacrificed in any way.
WO11/125956 and WO03/011967 disclose a method in which a photochromic curable composition is applied to a plastic lens by spin coating to be cured optically (to be also referred to as “coating method” hereinafter) and a method in which a photochromic curable composition is poured into a space between a plastic lens held by a elastomer gasket, adhesive tape or spacer and a glass mold to be polymerized and cured (to be also referred to as “two-stage polymerization method” hereinafter). Laminates having excellent photochromic properties can be manufactured by using these photochromic curable compositions.
However, due to growing demand for the improvement of the performance of photochromic plastic lenses, a photochromic curable composition from which a higher-performance lens than before can be manufactured has been desired.
To provide high color optical density and high fading speed which are the excellent properties of photochromic spectacles to a photochromic coating layer formed by the above coating method, the photochromic coating layer must be made soft. As a result, the mechanical strength of the photochromic coating layer lowers, whereby the photochromic coating layer is easily scratched in the step of processing a lens, for example, the step of polishing the rear surface of a lens to obtain desired power and the step of trimming the periphery of a lens in accordance with a frame form.
To cope with the above problem, the applicant previously proposed a photochromic curable composition which comprises a silsesquioxane compound forming a hard structure to achieve mechanical strength and photochromic properties at the same time (refer to WO13/008825).
However, as a result of further studies conducted by the inventors of the present invention, when the above photochromic curable composition comprising a silsesquioxane compound is stored for a certain period of time, it was found that the phase separation and clouding of the composition may occur, an appearance defect such as cracking readily occurs in the surface of a photochromic coating layer formed by using this photochromic curable composition, and satisfactory surface hardness is not obtained. Therefore, it was revealed that there is room for the improvement of the storage stability of the above photochromic curable composition comprising a silsesquioxane compound after it is stored for a long time.